Method and apparatus for controlling a patient&#39;s body temperature by in situ blood temperature modification

ABSTRACT

The present invention provides a method and apparatus for controlling the internal body temperature of a patient. According to the present invention, a catheter is inserted through an incision into a large blood vessel of a patient. By selectively heating or cooling a portion of the catheter lying within the blood vessel, heat may be transferred to or from blood flowing within the vessel and the patient&#39;s body temperature may thereby be increased or decreased as desired. The invention will find use in treating undesirable conditions of hypothermia and hyperthermia, or for inducing a condition of artificial hypothermia when desired.

This is a continuation of application Ser. No. 08/015,774, filed Feb.10, 1993, now abandoned.

BACKGROUND Of THE INVENTION

1. Field of the Invention

The present invention relates generally to the selective modificationand control of a patient's body temperature. More particularly, thepresent invention provides methods and apparatus for treatinghypothermia or hyperthermia by inserting a catheter into a blood vesselof the patient and selectively controlling the temperature of a portionof the catheter within the blood vessel. Heat is transferred to or fromblood flowing through the vessel and the patient's body temperature maythereby be increased or decreased as desired.

2. Description of the Background Art

Under ordinary circumstances the thermoregulatory system of the humanbody maintains a near constant temperature of about 37° C. (98.6° F.).Heat lost to the environment is precisely balanced by heat producedwithin the body.

Hypothermia is a condition of abnormally low body temperature.Hypothermia can be clinically defined as a core body temperature of 35°C. or less. Hypothermia is sometimes characterized further according toits severity. A body core temperature in the range from 32° C. to 35° C.is described as "mild" hypothermia, 30° C. to 32° C. is called"moderate," 24° C. to 30° C. is described as "severe," and a bodytemperature less than 24° C. constitutes "profound" hypothermia.Although the above ranges provide a useful basis for discussion, theyare not absolutes and definitions vary widely in the medical literature.

Accidental hypothermia results when heat loss to the environment exceedsthe body's ability to produce heat internally. In many cases,thermoregulation and heat production are normal but the patient becomeshypothermic due to overwhelming environmental cold stress. This is arelatively common condition, often resulting from exposure to theelements. Hypothermia may also occur in patients exposed to mild coldstress whose thermoregulatory ability has been lessened due to injury orillness. For example, this type of hypothermia sometimes occurs inpatients suffering from trauma or as a complication in patientsundergoing surgery.

Hypothermia of either type is a dangerous condition which can haveserious medical consequences. In particular, hypothermia interferes withthe ability of the heart to pump blood. Hypothermia may be fatal forthis reason alone. Additionally, low body temperature seriouslyinterferes with the enzymatic reactions necessary for blood clotting.This sometimes results in bleeding that is very difficult to control,even when normal clotting factor levels are present. These effects andother adverse consequences of hypothermia lead to drastically increasedmortality rates both among victims of trauma and in patients undergoingsurgery.

Simple methods for treating hypothermia have been known since very earlytimes. Such methods include wrapping the patient in blankets,administering warm fluids by mouth, and immersing the patient in a warmwater bath. Even these simple methods may be effective if thehypothermia is not too severe. These simple methods are limited in theireffectiveness however. Wrapping the patient in blankets ultimatelydepends on the patient's own production of heat to rewarm his body. Ineven moderate cases of hypothermia, or in the case of an ill or injuredpatient, the patient may simply be too weak or exhausted to producesufficient heat. Oral administration of a warm fluid requires that thepatient be conscious and capable of swallowing the fluid. Since loss ofconsciousness occurs early in hypothermia, this method is also limitedto moderate cases. Finally, immersion of the patient in a warm waterbath is often simply impractical. For example, immersion of a patientundergoing surgery would obviously be undesirable. Furthermore, theimmersion technique is time consuming and may be ineffective in that itrequires the transmission of warmth from the patient's skin surface intothe body core before the benefit of the warmth can be realized.

For this reason, methods have been devised to allow for the directwarming of a patient's blood. These methods involve removing blood fromthe patient, warming the blood in external warming equipment, anddelivering the blood back into the patient. While such methods are muchmore effective than any of the simple methods previously described, theyare disadvantageous for other reasons. First, the apparatus involved isquite cumbersome. Second, some danger is involved in even the temporaryremoval of significant quantities of blood from an already weakenedpatient. In fact, a further drop in body temperature is oftenexperienced when blood is first removed for warming in the externalapparatus. It would be desirable for these reasons to provide a methodand apparatus for directly warming blood in situ, i.e., within thepatient's body.

Hyperthermia, a condition of abnormally high body temperature, mayresult from exposure to a hot environment, overexertion, or fever. Bodycore temperatures can range from 38° C.-41° C. due to fever and may besubstantially higher in cases of exposure and overexertion. Likehypothermia, hyperthermia is a serious condition and can be fatal. Alsolike hypothermia, simple methods for treating hyperthermia, for example,immersion of the patient in a cool water bath or administration of coolfluids, have long been known. Generally, these simple methods fortreating hyperthermia suffer from the same drawbacks and limitedeffectiveness as the simple hypothermia treatments noted above.

It would therefore be desirable to develop more effective methods forlowering the body temperature of hyperthermic patients. Furthermore, itis sometimes beneficial to induce an artificial low-temperaturecondition (induced hypothermia) within a patient by artificial cooling.This may be desirable, for example, to reduce a patient's requirementfor oxygen during surgery or during a condition of cardiovascularcollapse.

To achieve these goals, methods have been used in which a patient'sblood is removed from his body, cooled in external cooling apparatus,and returned to his body. This external cooling suffers from the samedisadvantages as the external warming previously described. Externalcooling requires cumbersome apparatus and the temporary removal of bloodentails some degree of risk to the patient. It would therefore bedesirable to devise a method and apparatus for cooling blood within thepatient's body.

SUMMARY OF THE INVENTION

The present invention provides methods and apparatus for modifying andcontrolling a patient's body temperature. According to the presentinvention, a catheter is inserted percutaneously into a blood vessel ofthe patient. By controlling the temperature of a portion of the catheterlying within the blood vessel, heat may be selectively transferred to orfrom blood flowing through the vessel. The patient's body temperaturemay thereby be increased or decreased as desired. Some embodiments ofapparatus suitable for practicing the present invention will providemeans for treating hypothermia by warming a patient's blood. Otherembodiments will provide means for treating hyperthermia or inducing adesired condition of hypothermia by cooling the patient's blood.

Because blood circulates rapidly through the vascular system, thebeneficial effect of warming or cooling blood within the vessel will bequickly felt throughout the patient's body. In situ modification ofblood temperature is further advantageous in that blood is not removedfrom the patient. Additionally, no external pump is needed to circulatethe blood. Injury to blood components from the pump is therebyeliminated. Furthermore, the required apparatus is much simpler, lesscumbersome, and easier to use than the external blood warming or coolingapparatus previously known.

A catheter suitable for practicing the present invention will includemeans for warming or cooling at least a portion of the catheter insertedinto the blood vessel. It is desirable that such a catheter have arelatively small cross-section so as not to unnecessarily impede bloodflow through the vessel. On the other hand, a large heat transfersurface area will facilitate rapid heat transfer between the catheterand the blood. Structural features may therefore be included to increasethe surface area of the temperature controlled region.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a catheter according to the present invention insertedpercutaneously into a blood vessel of a patient;

FIG. 2 depicts a catheter suitable for increasing the temperature of apatient's blood by electrical resistance heating;

FIG. 3 depicts the distal end of a catheter having a resistance heatingelement and a temperature sensor;

FIG. 4 depicts the distal end of a catheter having an optical wave guideand an optical diffusing tip for converting laser energy into heat;

FIG. 5 depicts a catheter in which heat is transferred down a thermallyconductive shaft between the distal end of the catheter and heating orcooling apparatus at the proximal end of the shaft;

FIG. 6 depicts a catheter in which a heated or cooled fluid flowsthrough a balloon, which provides for an increased surface area at thedistal end;

FIG. 7 depicts a catheter having a resistance heating element at itsdistal end and a balloon having longitudinal ribs to further increasethe heat transfer surface area;

FIG. 8A depicts a catheter having longitudinal fins at the distal end ofthe catheter body;

FIG. 8B depicts a catheter having radial ribs at the distal end of thecatheter body; and

FIG. 8C depicts a catheter having a spiral fin to increase the heattransfer area at the distal end of the catheter.

FIG. 9 depicts a catheter having a balloon which is heated by currentflowing into two electrodes.

DESCRIPTION OF SPECIFIC EMBODIMENTS

The present invention provides methods and apparatus for selectivelymodifying and controlling a patient's body temperature by warming orcooling the patient's blood in situ. According to the present invention,a catheter is inserted through a puncture or incision into a bloodvessel in the patient's body. By warming or cooling a portion of thecatheter, heat may be transferred to or from blood flowing within thevessel and the patient's body temperature may thereby be increased ordecreased as desired. During the procedure, the patient's body coretemperature may be independently monitored and treatment may continueuntil the patient's core temperature approaches the desired level,usually the normal body temperature of about 37° C. Such methods willfind use in treating undesirable conditions of hypothermia andhyperthermia and may also be used to induce an artificial condition ofhypothermia when desired, e.g., to temporarily reduce a patient's needfor oxygen. In such a case, the patient's temperature may be reducedseveral degrees Celsius below the normal body temperature.

FIG. 1 depicts a distal end 15 of a catheter 10 according to the presentinvention. The catheter has been inserted through the patient's skininto a blood vessel BV. Blood flow through the vessel is indicated by aset of flow arrows F. Preferably, the catheter will be inserted into arelatively large blood vessel, e.g., the femoral artery or vein or thejugular vein. Use of these vessels is advantageous in that they arereadily accessible, provide safe and convenient insertion sites, andhave relatively large volumes of blood flowing through them. In general,large blood flow rates facilitate quicker heat transfer into or out ofthe patient.

For example, the jugular vein may have a diameter of about 22 French, ora bit more than 7 millimeters (1 French=0.013 inches=0.33 mm). Acatheter suitable for insertion into a vessel of this size can be madequite large relative to catheters intended for insertion into otherregions of the vascular system. Atherectomy or balloon angioplastycatheters are sometimes used to clear blockages from the coronary arteryand similar vessels. These catheters commonly have external diameters inthe range between 2 and 8 French.

In contrast, it is anticipated that a catheter according to the presentinvention will typically have an external diameter of about 10 French ormore, although this dimension may obviously be varied a great dealwithout departing from the basic principles of the claimed invention. Itis desirable that the catheter be small enough so that the puncture sitecan be entered using the percutaneous Seldinger technique, a techniquewell known to medical practitioners. To avoid vessel trauma, thecatheter will usually be less than 12 French in diameter upon insertion.Once in the vessel however, the distal or working end of the cathetercan be expanded to any size so long as blood flow is not unduly impeded.

Additionally, the femoral artery and vein and the jugular vein are allrelatively long and straight blood vessels. This will allow for theconvenient insertion of a catheter having a temperature controlledregion of considerable length. This is of course advantageous in thatmore heat may be transferred at a given temperature for a catheter of agiven diameter if the length of the heat transfer region is increased.

Techniques for inserting catheters into the above mentioned bloodvessels are well known among medical personnel. Although the method ofthe present invention will probably be most commonly employed in ahospital, the procedure need not be performed in an operating room. Theapparatus and procedure are so simple that the catheter may be insertedand treatment may begin in some cases even in an ambulance or in thefield.

The distal end 15 of the catheter may be heated or cooled as desired andheld at a temperature either somewhat above or somewhat below thepatient's body temperature. Blood flowing through the vessel willthereby be warmed or cooled. That blood will be circulated rapidlythroughout the patient's circulatory system. The beneficial effect ofwarming or cooling the patient's blood in the vicinity of the catheterwill thereby be spread very quickly throughout the entire body of thepatient.

FIGS. 2 and 3 depict a catheter suitable for treating hypothermia byincreasing the temperature of a patient's blood. As depicted in FIG. 2,the catheter has a preferably flexible catheter body 20. Disposed withinthe catheter body are a pair of electrical conduction leads 22 and 23and a temperature measurement lead 25.

Electrical conduction leads 22 and 23 are connected to a resistanceheating element 28, as depicted in FIG. 3. Electrical current providedby a power source (not shown) is converted to heat within the heatingcoil. That heat warms distal end 15 of the catheter and is therebytransferred to blood flowing through the vessel.

Temperature measurement lead 25 is connected to a temperature sensor 30.The temperature sensor facilitates the control of current flow throughthe heating coil. It is important to closely monitor the temperature ofthe distal end of the catheter and thus the flow of heat into thepatient's blood. Care must be taken not to overheat the blood whilestill providing an adequate rate of heat transfer into the patient. Theprovision of a sensitive temperature sensor at the distal end of thecatheter will help to achieve this goal.

FIG. 4 depicts an alternate embodiment of a catheter having means fortransferring energy from an external power source to distal end 15 ofcatheter body 20. In this embodiment, laser energy from a laser lightsource (not shown) is transmitted along optical wave guide 35. The waveguide directs the laser energy into optical diffusing tip 37, whichconverts the laser energy to heat. From diffusing tip 37, the heatradiates outward into distal end 15 of the catheter and from there intothe patient's blood stream.

FIG. 5 depicts another catheter suitable for practicing the presentinvention. This embodiment has a thermally conductive shaft 40 runningthe length of catheter body 20. Shaft 40 is made of a metal or othermaterial having a high thermal conductivity. By heating or cooling theproximal end 42 of shaft 40 with an external heating or coolingapparatus 45, heat will be caused to flow either into or out of thedistal end 47 of the shaft. In the embodiment depicted, the distal endof the shaft is fitted with heat transfer vanes 50, which add to thesurface area of the shaft and thereby promote more effective heattransfer between the catheter and the patient's blood stream.

FIG. 6 depicts still another means for transferring heat to or from thedistal end of a catheter. In this embodiment, catheter body 20 has twolumens running through it. Fluid flows from the proximal end of thecatheter through in-flow lumen 60, through a heat transfer region 62,and back out through out-flow lumen 64. By supplying either warmed orcooled fluid through inflow lumen 60, heat may be transferred either toor from the patient's blood stream.

In the embodiment depicted, heat transfer region 62 is in the form of aballoon 70. Use of a balloon will be advantageous in some embodiments toprovide an increased surface area through which heat transfer may takeplace. Balloon inflation is maintained by a pressure difference in thefluid as it flows through in-flow lumen 60 and out-flow lumen 64. Theballoon should be inflated to a diameter somewhat less than that of theinside diameter of the blood vessel so as not to unduly impede the flowof blood through the vessel.

FIG. 7 depicts a catheter having an internal resistance heating element28 and a balloon 70, which is shown inflated. In this embodiment, theincreased surface area provided by the inflated balloon is furtheraugmented by the presence of a set of longitudinal fins 75 on thesurface of the balloon. Alternatively, longitudinal fins 75, radial ribs77, or one or more spiral fins 79 may be disposed directly on the body20 of a catheter as shown in FIGS. 8A, 8B and 8C. Ordinarily,longitudinal ribs will be most advantageous because they restrict bloodflow through the vessel less than other configurations. In fact, theseribs insure that the balloon will not block the flow of blood throughthe vessel because a flow path will always be maintained (between theribs) regardless of how much the balloon is inflated.

Inclusion of a balloon on a catheter employing resistance heating allowsfor designs in which current is conducted through the fluid which fillsthe balloon. The catheter depicted in FIG. 9 has a catheter body 20about which is disposed an inflatable balloon 70. The balloon isinflated by injecting a suitable fluid into the balloon through centralballoon inflation lumen 80. In this embodiment, current flows from anexternal source of electrical power (not shown) through conduction wires82 and 84 to electrodes 86 and 88.

A suitable fluid will allow current to flow between electrodes 86 and88. Common saline solution, for example, contains dissolved ions whichcan serve as charge conductors. Electrical resistance within the fluidwill cause the fluid to be heated, thus providing the desired warming ofthe catheter. The amount of warming will be dependant upon the voltagebetween the electrodes, the distance between them, and the resistivityof the fluid. The relation between these quantities is fairly simple;one skilled in the art will have no difficulty selecting appropriatevalues.

Resistance heating catheters like those depicted in FIGS. 3, 7 and 9 mayuse DC or low frequency AC power supplies. However, it may be desirableto use a higher frequency power supply. For example, it is known thatthe risk of adverse physiological response or electrocution response maybe lessened at frequencies within the range of about 100 kilohertz to 1megahertz. Power supplies that operate at these frequencies are commonlyreferred to as radio-frequency, or RF, power supplies.

A catheter according to the present invention should be designed tooptimize the rate of heat transfer between the catheter and bloodflowing through the vessel. While a large surface area is desirable inorder to maximize heat transfer, care must be taken so that the catheterdoes not unduly restrict blood flow through the vessel. Furthermore, thetemperature of the catheter should be carefully controlled to preventundesirable chemical changes within the blood. This is especiallyimportant when applying heat to the blood as blood is readily denaturedby even moderately high temperatures. The exterior temperature of acatheter for warming blood should generally not exceed about 42° C.-43°C.

It is estimated that a catheter whose surface temperature is controlledbetween 37° C. and 42° C. will provide a body core warming rate ofapproximately one to two degrees Celsius per hour in a patient startingout with severe hypothermia. This estimate is highly dependant on anumber of factors including the rate of blood flow through the vessel,the initial body temperature of the patient, the external surface areaof the catheter through which heat is conducted, etc. The actual rateachieved may vary substantially from the above estimate.

The above estimate provides a starting point for a rough estimate as tothe level of power transferred from the catheter to the patient's bodyand therefore of the size of the power supply required by the system.Regardless of the exact means of power transmission chosen, resistanceheating coil, laser and diffusing tip, direct conduction or fluidcirculation, an appropriate power supply will be required to provideheat to the system.

The sum of heat entering and leaving a patient's body can be written as:

    ΔH=H.sub.c +H.sub.i -H.sub.e

where ΔH is the sum of all heat transferred, H_(c) is the heattransferred from the catheter to the patient, H_(i) the heat produced bythe patient internally, and H_(e) the heat lost from the patient to theenvironment. If one assumes, as will ordinarily be the case in a healthypatient, that the body's internal thermoregulatory system will producejust enough heat to offset heat lost to the environment, then theequation is made simple:

    ΔH=H.sub.c.

The above equation can be written in terms of the change in thepatient's internal body temperature over time as follows:

    mc(ΔT/Δt)=(ΔH.sub.c /Δt)

where m is the body mass of the patient, c is the specific heat of thepatient's body, (ΔT/Δt) is the time rate of change of the patient'sinternal body temperature, (ΔH_(c) /Δt) is the time rate of heatdelivery from the catheter to the patient.

If one assumes a patient having a body mass of 75 kilograms and aspecific heat of 4186 joules/°C.-kg (assumes the specific heat of thehuman body to be the same as that of water, the actual value will besomewhat different), then a warming rate of 1° C. per hour (3600seconds) will require the catheter to transfer heat to the patient at arate of about 87 watts (1 watt=1 joule/sec).

However, as an estimate of the desirable size of a power supply to beused with a catheter of the present invention, this estimate is almostcertainly too low. This is true for a number of reasons. First, it wasassumed for the sake of convenience that the patient's internal systemwould produce an amount of heat equal to that lost to the environment.In a hypothermic patient this will obviously not be the case. Almost bydefinition, hypothermia occurs when a person's ability to produce heatinternally is overwhelmed by heat lost to the environment. The catheterwill have to make up the difference so the power level required willneed to be greater for that reason alone.

Additionally, the above estimate does not allow for power losses betweenthe power supply and whatever warming means is utilized. Such lossescould include resistance losses in electrical transmission lines betweenthe power supply and a resistance heating element, inherentinefficiencies and other losses in a system having a laser and adiffusing tip, heat losses along a thermally conductive shaft or fluidcirculation lumen, and the like. Any such losses which do occur willneed to be compensated for by additional power supply capacity.

Furthermore, it would be undesirable to limit the performance of acatheter according to the present invention by limiting the size of thepower supply used. It would be preferable instead to use a power supplycapable of providing power considerably in excess of that actuallyneeded and then controlling the delivery of that power according to themeasured temperature of the catheter itself. As mentioned previously,this can be readily accomplished by including a sensitive temperaturesensor within the body of the catheter. Nevertheless, the abovecalculation can be used as a useful estimate of the likely lower boundfor sizing a power supply for use in a catheter according to the presentinvention.

An alternative estimate can be made by comparing the likely performanceof the various embodiments described herein with the power requirementsfor the external blood warming apparatus presently known. Such externalwarming apparatus generally requires a supply of power on the order of1000-1500 watts and sometimes more. A device according to the presentinvention will most likely require considerably less power than that.First, the present invention requires no external pump to circulate theblood; this function is provided by the patient's own heart.Accordingly, no power is needed to drive such a pump. Secondly, thepresent invention is considerably less complicated than external bloodwarming systems. Known systems circulate the blood over a relativelylengthy path from the patient, through the warming element, and backinto the patient. It is expected that more heat is lost over thislengthy path than will be lost in any device according to the presentinvention.

Thus, the power required by external blood circulation and warmingsystems of the type previously known can be used as a rough estimate ofthe likely upper limit for power required by a system according to thepresent invention. It is most likely that such a system will best beequipped with a power supply having a capacity somewhere between the tworough estimates described above. It is therefore contemplated that asuitable power supply will be capable of providing peak power somewherein the range between 100 and 1500 watts, probably being in the rangebetween 300 and 1000 watts. The ranges specified are an estimate ofsuitable peak power capability. The power supply will most commonly bethermostatically controlled in response to a temperature sensor in thebody of the catheter. The actual effective power transmitted to thepatient will therefore typically be much less than the peak powercapacity of the system power supply.

With respect to a catheter for cooling, the temperature and powerconstraints are not as limiting as is the case in a catheter for warmingblood. Care should merely be taken to avoid freezing the blood orinducing shock to the patient from too rapid cooling.

Blood is essentially water containing a number of suspended anddissolved substances. As such, its freezing point is somewhat below 0°C. However, a catheter adapted to cool blood in a hyperthermic patientor to induce an artificial hypothermia will usually not be operated attemperatures that low. It is presently contemplated that the externalsurface of such a catheter may be held in the range between about 20° C.and 24° C., although the actual temperature could vary between about 0°C. and the patient's current body temperature (somewhat in excess of 37°C.).

Various embodiments of apparatus suitable for practicing the methods ofthe present invention have been described. Other embodiments andmodifications will occur to those skilled in the art. For example,various means for heat transfer, e.g., resistance, including radiofrequency, heating; laser energy; pumped fluids; etc., may be combinedwith various means for increasing the effective heat transfer surfacearea, e.g., balloons, fins, ribs, etc., to optimize the function of adevice according to the present invention. Also, a temperature sensorwill typically be used although for ease of illustration such a sensoris not depicted in all of the embodiments described. Furthermore,although most of the figures depict embodiments in which only a limitedportion of the catheter is temperature controlled, no reason exists toprevent warming or cooling substantially the whole length of thecatheter.

Broadly stated, the present invention provides a method for modifying apatient's body temperature by controlling the temperature of a catheterinserted into a blood vessel of the patient. Although severalillustrative examples of means for practicing the invention aredescribed above, these examples are by no means exhaustive of allpossible means for practicing the invention. The scope of the inventionshould therefore be determined with reference to the appended claims,along with the full range of equivalents to which those claims areentitled.

What is claimed is:
 1. A method for increasing a patient's bodytemperature by warming blood within the patient, the method comprisingthe steps of:inserting a catheter having a heating element into a bloodvessel of the patient so that the heating element lies within the bloodvessel; providing heat transfer fins projecting radially outward fromthe catheter in the region of the heating element; transferring heat tothe blood through the heating element, wherein heat transfer is enhancedby the presence of the heat transfer fins.
 2. A method as in claim 1,wherein the heating element is a resistance heating element and heat istransferred by conducting electrical current through the resistanceheating element.
 3. A method as in claim 1, further comprising inflatinga balloon around the heating element with a heat transmissivefluid,wherein the heat transfer fins project radially outward from theballoon and the heat transmissive transfer fluid effects heat transferfrom the heating element to the heat transfer fins.
 4. A method as inclaim 1 wherein the heating element is heated to a temperature notexceeding 43° C. to transfer heat to the blood.
 5. A method for treatinga patient suffering from hypothermia by warming blood within thepatient, the method comprising the steps of:inserting a catheter havinga heating element into a blood vessel of the patient so that the heatingelement lies within the blood vessel; providing heat transfer finsprojecting radially outward from the catheter in the region of theheating element; transferring heat to the blood through the heatingelement to increase the patient's body temperature, wherein heattransfer is enhanced by the presence of the heat transfer fins.
 6. Amethod as in claim 5, wherein the heating element is a resistanceheating element and heat is transferred by conducting electrical currentthrough the resistance heating element.
 7. A method as in claim 5,further comprising inflating a balloon around the heating element with aheat transmissive fluid,wherein the heat transfer fins project radiallyoutward from the balloon and the heat transmissive fluid effects heattransfer from the heating element to the heat transfer fins.